Industry in general, and the automobile industry in particular, produces a large number of stamped sheet metal parts each year. With the greatly increased usage of the new lighter weight and higher strength alloys, the problems of producing such parts have become aggravated. The design of stamping dies is largely an art and the considerable body of experience relative to dies for stamping steel is not readily applied in connection with the newer materials. The high cost incurred in acquiring the needed information by the trial and error methods of die design necessitates a different approach. One promising approach is to develop a better understanding of the stamping process itself. A measurement frequently required in stamping studies is the amount of strain present in the metal at various locations. In addition to supporting research efforts to understand the metal forming process and die design, strain measurements are also useful as an inspection aid. The strength of metals can be correlated with the amount of strain. The maximum strain in critical parts can also be checked to determine how close they are to the failure strain.
The standard technique employed to record strain measurements is carried out as follows. Sheet stock is printed with grids of small circles prior to a forming operation. As the metal is formed, the circles stretch into ellipses in proportion to the strain. Because the circles were printed in a known size, it is possible to calculate the strain by measuring the dimensions of the ellipse patterns. The circles can be printed in a variety of sizes and configurations using a variety of printing methods. They may be printed as open rings or filled-in dots. Two commonly employed printing methods are the photo-resist and electro-etch techniques.
The measurement of many small ellipses is a very labor intensive operation. For quick, approximate measurements, a clear plastic mask is held over the sample and slid along the sample to match up the appropriate dimensions. If greater precision is required, the samples to be measured are cut into pieces which can be laid on a calibrated toolmaker's microscope stage. The microscope stage is fitted with a digital position read-out indicating the amount of stage translation. The grid circle to be measured is aligned along one of the table axes with one side centered beneath a cross hair. The position counter is then zeroed and the stage moved until the opposite side is aligned beneath the cross hair. The display then indicates the ellipse measurement in some arbitrary units. This method is currently being used and produces good results. Readings have been found to be accurate to plus or minus 0.3 percent of the true strain value. The measurement does, however, involve some subjectivity in deciding where the pattern edge lies and in precisely orienting the pattern along one of the principle axes. This is difficult to achieve in severe cases where the pattern is degraded due to the stamping process and also when the strain levels are low. A skilled technician requires approximately one to two minutes for each point measured. This technique normally requires two similar measurements--once for the major axis strain and again for the minor axis strain. When many points must be measured, it becomes a very tedious time consuming operation.